[RPG] Rules for physical appearances based on charisma and race

Player's Handbook II (3.5) / Divine Conversion / p193

This is a sidebar at the bottom of the referenced page, which is part of a section on rules for retraining. If you don't have the book, here is the direct quote:

DIVINE CONVERSION

As noted in the Player’s Handbook, a cleric who grossly violates the code of conduct imposed by his deity loses all spells and class features and cannot attain any more levels as a cleric of that deity. All these penalties remain in effect until he atones. But what if he doesn’t want to atone? What if a cleric of Hextor finds new meaning and purpose in serving Heironeous after a dramatic conversion experience? Such a character need not become a multiclass ex-cleric of Hextor/cleric of Heironeous. Instead, Heironeous can simply reinstate the character’s cleric powers once he has proven his loyalty, talent, and ability.

A cleric who changes his patron deity must complete a quest to prove his devotion to his new patron. The nature of the quest depends on the deity, and it always clearly reflects the deity’s alignment as well as his or her goals and beliefs. To start the process, the cleric must voluntarily accept a geas/quest spell cast by a higher-level cleric of his new deity. During the quest, the cleric has no access to spells or cleric class features—except his weapon and armor proficiencies, which he does not forfeit.

Upon completing the quest, the cleric receives the benefit of an atonement spell from a cleric of the new deity. The character then becomes a cleric of the new deity and is inducted into the clergy during an appropriate ceremony of the DM’s choosing. After selecting two of the new deity’s domains in lieu of his old ones, the character has all the powers and abilities of his previous cleric level, plus the granted powers of his new domains.

This method is the only one by which a cleric can change his deity. The retraining rules can’t be used to accomplish this task—it is simply too substantial a change in the character’s identity (not to mention his source of power) to chalk up to a bit of practice in his off hours.

This answer essentially reflects the ruling already found in the Forgotten Realms Campaign Setting, but it may be useful to know it's also located in a setting-independent rulebook, just in case there are any sticklers out there.

The simple answer is you only get to use one other source material book, beyond the PHB, for everything related to your character creation (and advancement). So no, you can't have a Zariel tiefling as a Conquest Paladin.

Per the ALPG v9.1:

Forgotten Realms characters can choose race and class options from the Player’s Handbook and one other resource—a rule called “PHB+1.”

Since you want your class to be the Paladin's Oath of Conquest, which comes from Xanathar, all other creation choices (except where spelled out) such as your race MUST come from either the PHB or XGtE.

For the the second part, the bulleted list refers to variants that are allowed.

• Variant Human Traits (PHB)
  • Since everyone gets the PHB, anyone can use the variant human traits
• Half-Elf and Tiefling Variants (SCAG/ToF)
  • If SCAG or ToF is your +1, then you can use the variant versions mentioned in that source
• Option: Human Languages (SCAG) (3)
  • Optional Human Languages are mentioned in SCAG, but (3) says you can take this even if SCAG is not your +1
• Blessing of Corellon (ToF) (3)
  • Even if ToF is not your +1, you can still apply the optional Blessing to your elf

It's a really horribly worded doc, and it gets worse every revision as they try to shoehorn in new rules to existing text. Such as, in previous versions of the doc, they specifically called out variants and optional play were not allowed. But then they realized that feats and multi-classing are listed as optional and therefor technically are against the rules. So they took that out, but didn't fix surrounding text.